The present invention relates generally to liquid crystal display devices; and, more particularly, the invention relates to a liquid crystal display device that maintains a uniform gap for a liquid crystal layer, to a liquid crystal display device that can prevent optical leakage in the display area, and to a liquid crystal display device that can prevent pollution in the liquid crystal composite material by the seal agent.
Recently, display devices using a liquid crystal panel have become more widely employed as display devices which are capable of visually producing high-precision color images adaptable for use in display devices of the projection type, in notebook personal computers, in monitor units and in other similar visual representation instruments.
Currently available display devices using such a liquid crystal panel (liquid crystal display devices) typically include those of the simple matrix type, which make use of a liquid crystal panel having a liquid crystal layer sandwiched between a pair of substrates with parallel electrodes formed on respective inner surfaces in a mutual crossover fashion, and other panels of the active matrix type which employ a liquid crystal element (referred to also as xe2x80x9cliquid crystal panelxe2x80x9d hereinafter) that has switching elements for selection in units of pixels on only one of the pair of substrates.
Active-matrix liquid crystal display devices are generally categorized into two groups: one group includes certain liquid crystal display devices of the so-called vertical electric-field type typically including the twisted nematic (TN) scheme (also known as TN active-matrix liquid crystal display devices) configured to include an ensemble of pixel selection electrodes formed on each of a pair of upper and lower substrates, and a second group includes the so-called xe2x80x9clateral electric-fieldxe2x80x9d liquid crystal display devices (generally called in-plane field type IPS liquid crystal display devices) using a specific liquid crystal panel with pixel select electrodes formed on only one of a pair of upper and lower substrates.
Projection liquid crystal display devices are also known as one type of liquid crystal display device application equipment. Projection liquid crystal display devices include an optical system for magnification of an image generated on a liquid crystal panel of small size to provide an enlarged image which is then projected onto a spaced-apart second screen of large size. Such projection liquid crystal display devices include devices of the transmission type and those of the reflection type, the former being designed such that two separate dielectric substrates making up a liquid crystal panel are both formed of transparent substrates, such as glass substrates by way of example, for permitting rays of light to be emitted from the back surface thereof to thereby cause resulting modulated transmission light images to be projected with enlarged sizes on an associative screen by use of an optical lens or combination thereof. On the other hand, the reflective projectors employ one of such dielectric substrates as a reflector plate for emitting light from the surface side to thereby produce an image which consists of modulated reflected light, which in turn is projected by an optical system on a screen with a magnified scale.
There are also display devices for use with notebook PCs or direct view liquid crystal display devices for display monitors, which are designed to employ as a reflector plate either one of the dielectric substrates making up the liquid crystal panel and which utilizes incoming light from the display surface side.
Typically, a liquid crystal panel constituting such a liquid crystal display device is arranged so that a liquid crystal layer made of a chosen liquid crystal material is sandwiched in a gap between two separate dielectric substrates which are bonded together, such as glass substrates, for example, and thereafter the peripheral edges thereof are sealed using a chosen seal material. The gap between two dielectric substrates is narrow and typically will measure less than 4 to 7 micrometers (xcexcm) for instance, which gap will be collectively referred hereinafter as a xe2x80x9ccell gapxe2x80x9d. One prior known method of retaining this cell gap is to randomly distribute spherical spacers of substantially uniform diameter, sometimes called beads, between the substrates.
Although controllability of the cell gap may readily be enhanced by increasing the requisite number of beads that are distributed, the distribution amount has generally been set at 150 pieces per square millimeter in view of the fact that random distribution of such beads inherently lacks uniformity thereby making it very difficult to completely prevent some beads from locally crowding together at a location. This can result in an increase in the number of optical dot-like dislocations, and the random bead distribution also causes an adverse reaction, such as creation of an undesired disturbance in the alignment of the liquid crystals near or around such beads, which would result in a contrast reduction becoming greater locally.
While the beads may be made of an organic polymer or quartz, use of quartz beads can cause destruction of any one of the protective films, the electrodes, and the switching elements, such as TFTs, which are fabricated on a dielectric substrate at a press-machining step for establishment of the cell gap, or alternatively result in unwanted creation of air holes or xe2x80x9cbubblesxe2x80x9d with a change in temperature due to a difference in the thermal expansion coefficient between the beads and a liquid crystal material being used. For this reason organic polymer beads are employed in most cases.
In direct-view liquid crystal display devices, the beads which are distributed often attempt to move or xe2x80x9cdriftxe2x80x9d upon application of a stress to the dielectric substrate. In this respect, it will be desirable for the liquid crystal layer to be kept at negative pressures relative to the atmospheric; however, presently available manufacturing technologies make it difficult to constantly maintain such a state in which the liquid crystal panel products are constantly held in a negative pressure condition.
On the other hand, small size liquid crystal display devices for use in projector equipment are burdened with a problem in that certain beads distributed between dielectric substrates of its liquid crystal panel, which reside in the panel""s display area, can unintentionally be projected on a screen as a magnified shadow image, which in turn results in a decrease in the quality of the picture images being displayed. One prior known approach to avoiding such image quality reduction is to employ what is called a xe2x80x9cbeads-lessxe2x80x9d scheme which uses a limited number of beads or fibers only at the periphery of the liquid crystal panel""s display area to thereby retain the intended cell gap at such periphery only. Unfortunately, this beads-less approach suffers from a difficulty in maintaining the cell gap in the display area at a predetermined value, which can result in a decrease in the production yield and in image quality.
Further, in recent years, high-speed image displayability has been demanded, which in turn calls for establishment of so-called xe2x80x9cnarrow gapxe2x80x9d designs for further reduction of cell gaps with increased gap control accuracies of 0.1 xcexcm or below. As such narrow-gap designs are becoming more important, a need is felt to further increase the bead-spacer machining accuracy, which however is very difficult, especially in prior art reflective liquid crystal panels, wherein achievement of such high machining accuracy remains extremely difficult due to the fact that the cell gaps are nearly half the size of those in the devices of the transmission type.
One proposed approach to avoiding the cell-gap problem is to form, by photolithography techniques, columnar or pillar-shaped spacers (referred to hereinafter as xe2x80x9cpole-like spacersxe2x80x9d) on a dielectric substrate at selected locations (certain portions that do not affect displayability, such as portions between adjacent pixels or alternatively those immediately underlying a black matrix) in the display area thereof, which spacers provide support between the two dielectric substrates stacked over each other to thereby render the cell gap uniform.
Use of such pole-like spacers eliminates local crowding and unwanted drift movement of distributed beads. Furthermore, as the fabrication accuracy of photolithography is significantly greater than the machining accuracy of beads by one order of magnitude or greater, the height of the pole spacers is simply determinable depending upon the thickness of the deposited photoresist film constituting these pole spacers, which in turn makes it possible to noticeably improve the cell gap accuracy.
Unfortunately, currently available photoresist materials can dissolve into a liquid crystal material, so as to undesirably reduce the electrical resistivity of the liquid crystal layer, which would result in a decrease in co-useability or xe2x80x9ccongenialityxe2x80x9d with respect to the liquid crystal materials. Alternative use of inorganic materials therefor can result in a mismatch of the thermal expansion coefficient with the liquid crystal layer. All in all, no optimal materials adaptable for use in fabricating the intended spacers have been reported to date.
One typical prior known approach to controlling the cell gap is to mix either fibers or beads made of organic polymer or quartz as a filler into a seal material being deposited at the outer periphery of a display area. However, this approach also creates a problem in that the use of a quartz filler(s) can result in destruction of the lead terminal electrodes and/or switching circuitry, as in the case of employing beads distributed within the display area. While it is also considered effective to coat a seal material at specific portions lying outside of a switching circuitry formation region of the display area, this inherently creates a serious problem in that the resulting liquid crystal panel increases in size due to a need to reserve an extra area for seal portions. Another problem encountered in the prior art is difficultly in improving the accuracy of the cell gap because of the fact that organic polymer beads are readily collapsible; and, in view of this, it is a general approach to employ fibers for the seal portions.
A known sealing method includes the steps of coating, by use of screen printing techniques or using dispensers or the like, one of two dielectric substrates with a filler-mixed seal material in a selected region along the outer periphery of its display area, laminating the other dielectric substrate over the seal material-coated substrate, pressing these substrates together at increased pressures to permit the seal material sandwiched therebetween to sheet against the substrate surface for establishment of the intended cell gap, and then hardening the seal material sheet. A disadvantage of this method is that it remains impossible, or at least greatly difficult, to attain the required accuracy of position alignment at the seal edge portions, which results in the seal portions becoming irregular in shape. An optical blocking or shielding means must be additionally provided to preclude unintentional visualization of such an irregular seal edge shape. Especially with small size liquid crystal panels, use of such optical shield means can result in an increase in the surface area for use in sealing.
An object of the present invention is to provide an improved liquid crystal display device which is capable of eliminating display irregularities by avoiding random behavior (either local crowing or drift movement) of beads in a display area which can occur when using beads, as well as destruction of switching elements and electrodes or the like due to presence of fillers contained in beads or seal materials, along with cell gap differences in the liquid crystal panel occurring in the display area and at sealed portions, while at the same time enabling achievement of high-quality displayability by precluding contamination of a liquid crystal material due to unwanted contact between the seal material and the liquid crystal layer.
To attain the foregoing object the present invention, spacpes are formed photolithographically both in a display surface area of one dielectric substrate of a liquid crystal panel constituting the liquid crystal display device and at sealed portions thereof at the same time. The spacers formed within the display area are comprised of columnar or pole-like spacers while those formed at the sealed portions consists of a zonal or band-shaped spacer which has a width which is greater than the diameter of such pole spacers. A chosen sealing material containing no fillers therein is deposited or coated at the outer periphery of this zonal spacer, which material is later hardened, thus allowing both substrates to be tightly bonded together.
With regard to the embodiments disclosed herein, some representative aspects of the invention will be summarized below.
A liquid crystal display device in accordance with the instant invention is arranged to include a first substrate having thereon a great number of pixel electrodes in the form of a matrix, a second substrate opposing said first substrate with a predefined gap defined between them, a liquid crystal layer made of a liquid crystal composition material sealed into the gap between said first and second substrates, and an optical alignment film formed on at least one of said first and second substrates in contact with said liquid crystal layer for controlling the optical orientation or alignment of said liquid crystal material, wherein the device further includes a plurality of columnar or pole-like spacers formed in the display surface area of said first substrate for retaining the size of said gap between said first and second substrates at a preselected value, while also including a zonal or band-shaped spacer made of the same material as that of said pole-like spacers for surrounding said display area and having a width greater than the diameter of said pole-like spacers, with a seal material being filled at the outer periphery of said strip-like spacer for tightly bonding said first and second substrates together. Note that providing the band spacer avoids the necessity for the seal material to contain therein beads or fibers or any equivalents thereto for use in controlling the gap between the two substrates.
With such an arrangement, the spacer""s inherent random behavior within the display area may be suppressed or eliminated, thereby retaining a more uniform resultant cell gap. Another advantage is that the seal material will no longer come into contact with the liquid crystal layer thus precluding contamination of the liquid crystal material due to the presence of seal material, which in turn makes it possible to avoid destruction of the electrodes and the like due to the beads in the display area or alternatively destruction of electrode extension leads and the like at the sealed portions due to presence of fillers mixed into the seal material, thus improving the production yields and the reliability.
A further advantage is that the pole spacers stay equal in height to the band spacer with an increased accuracy to thereby enable the cell gap to be well controlled to a high accuracy over almost all regions of the display area, which in turn makes it possible to eliminate visualization irregularities during displaying of on-screen images, including flutter, moire, streaking, and pixel jitter at certain intensities.
Additionally the present invention should not be limited only to the above-noted arrangements and may alternatively be modified and altered in a variety of different forms without departing from the technical concept of the invention.